This invention concerns an exhaust system for a motor vehicle equipped with a diesel propulsion engine, the exhaust system of which comprises a discontinuously regenerating exhaust gas purification system. Discontinuously regenerating exhaust gas purification systems include components such as diesel particulate filters and NOx-accumulating catalytic converters.
To comply with environmental specifications, exhaust gases of motor vehicles propelled by combustion engines are subjected to a purification process. Specifically, appropriate particulate filters are utilized to reduce particulate emissions of exhaust gases of motor vehicles propelled by diesel engines. Such particulate filters must be periodically regenerated by burning off particles accumulated on a filter surface. Due to the relatively low exhaust gas temperatures of modern diesel engines, to initiate regeneration, the exhaust gas temperature must be raised by approximately 100° C., using appropriate procedures or devices. The exhaust gas temperature must be raised if filter regeneration is to be feasible in conditions other than full load operation, even if fuel additives are used that are capable of lowering particulate ignition temperature. Without such additives, depending on the soot composition, the particulate ignition temperature ranges between 470° C. and 600° C. However, the use of such additives, in the long run, can cause the particulate filter to clog, necessitating its cleaning.
For this purpose various burners (refer, e.g., to DE 19504183 A1 and DE 19717544 A1) have been proposed for installation upstream of the particulate filter. Some disadvantages with such burners are the considerable structural and financial efforts associated with such burners, the considerable increase in space required due to the integration of a relatively large burner into the exhaust system, and the detrimental effect on the flow conditions within the exhaust pipe.
One solution, which relative to some of the above mentioned aspects, is more favorable, has been proposed in EP 132166 A1. In EP 132166, a low-boiling, organic fluid evaporates at a glow plug that extends into an exhaust gas carrying component located upstream of the particulate filter. Vapors either ignite at the tip of the glow plug after their thorough mixing with the oxygenous exhaust gas or, without any ignition, are mixed into the stream of exhaust gases. In the latter case a temperature increase of the exhaust gases ducted to the particulate filter, is obtained by catalytic oxidation of the vapors of the organic fluid in a catalytically coated area of the particulate filter. A particular disadvantage of this is the engineering and logistic effort associated with the storage of the low boiling organic fluid. Furthermore, the regeneration of the diesel particulate filter utilizing this heating device calls for exhaust gas temperatures of above 450° C., which in modern diesel engines can only be attained under full load.
A comparable approach is pursued by DE 19533355 A1 in which the temperature of the exhaust gases is raised for the purpose of regenerating a particulate filter by catalytically burning an auxiliary fuel, particularly a fuel gas, in an oxidizing converter connected upstream of the filter, utilizing the remaining oxygen in the exhaust gas stream and essentially without any additional combustion air. The disadvantages arising from it are the same as described before in conjunction with EP 132166 A1.
DE 3139565 A1, from which a motor vehicle of a generic kind may be derived, describes using atomizer nozzles to inject diesel fuel into, or immediately upstream of, a catalytically coated area of a particulate filter to increase the exhaust gas temperature in this manner by means of catalytic oxidation of the fuel. An electrical heating element is embedded in the catalytically coated area of the particulate filter to initiate catalytic oxidation. Specific disadvantages of the DE 3139565 A1 system are, aside from a considerable non-homogeneity of the temperature distribution, the fact that due to the partial evaporation the atomized fuel draws heat from the exhaust gas stream, and that the atomizer nozzles—given the conditions prevailing in non-purified exhaust gases of a diesel engine—tend to clog, thereby rendering the respective heating device non-functional within a short period of time. Thus, systems based on DE 3139565 A1 were never really utilized.
Finally, attempts have been made to increase the exhaust gas temperature up to the ignition temperature of the soot collecting on the particulate filter, using electrical heating elements. These attempts were quickly aborted as the required electrical power was not available in conventional motor vehicles.
NOx-accumulating catalytic converters are increasingly used to reduce the emission of nitrogen oxides. The regeneration of these converters requires an enrichment of the exhaust gases. While this is not a problem with gasoline engines, diesel engines that operate with excess air and whose exhaust gases therefore, as a rule, also contain excess air, require special procedures.